One of the most important potential uses of embryonic stem cells and induced pluripotent stem cell research is in the development of disease models. Although many diseases can be modeled in mice, others are difficult or impossible to adequately model in non-human organisms for a variety of reasons. In the retina, for example, there are mouse models for many inherited degenerations, but these have been more difficult to generate for diseases like macular degeneration and many forms of Usher's disease. Therefore, disease models from human patient iPS cells would be of great utility in retinal research. In the past six years, our group, and others, have developed protocols for efficient production of retinal cells from human ESCs and iPSCs. The early stages of retinal development are extremely well recapitulated by our protocol of directed differentiation of hESC cells; however, the cells fail to attain the level of expression of markers that we observe in the late stages of human fetal or postnatal development, even with prolonged culture periods. Therefore, a significant challenge for creation of disease models from iPSCs will be to better understand and promote the progression of the cultured cells to mature stages of retina. The mechanisms that control the developmental timing of retinal progenitor cells are not clear; however, recent evidence from our lab shows a key role for miRNAs. Specifically, we found that the loss of Dicer in retinal progenitor cells leads to their failure to progress from the early state to the late state. We therefore hypothesize (1) that miRNAs regulate developmental timing in retinal progenitor cells and (2) that misregulation of the heterochronic pathway in ESC-derived retinal cells accounts for their failure to progress in vitro at the same rate as in vivo. We propose to test these hypotheses with the following specific aims. Aim 1: Determine whether the expression of specific miRNAs (and the genes they regulate) correlates with the progression of retinal progenitors from the early to late stage. Aim 2.Determine whether the developmental progression of mouse ESC/iPSC-derived retinal progenitors is controlled by stage-specific progenitor miRNAs. Aim 3. Determine whether the developmental progression of human ESC derived retinal progenitors can be accelerated by stage-specific miRNAs. The results of these studies will enable us to better control the development of hESC-derived retinal cells and produce more appropriate models of retinal disease.